# Photons determine the property of an electromagnetic wave

1. Nov 25, 2008

### FrankJ777

Hi all,
I have some questions about photons. I'm at the end of the semester of PhysII in the quantum mechanics section and I'm trying to understand how photons determine the property of an electromagnetic wave. The way the text book describes it the electromagnetic waves are made up of photons, and the frequency of light or the wave is actually the frequency of photons themselves. Or is it that the waves are made up of photons and thre frequency of the wave is determined by varying density of photons? I guess i'm confussed especially when it comes to radio waves. If a wave is propagated because of an oscillation of current how can an antenna emmit a continuous "stream" of photons at that frequency of current oscillation?

2. Nov 25, 2008

### ice109

Re: photons

photons are the quanta of the electromagnatic field. what that means will take knowing quantum field theory. i don't know it yet but i do know that saying that an electromagnetic wave is made up of photons isn't "grammatically" correct. i wouldn't worry about it too much until you get to quantum field theory.

3. Nov 26, 2008

Re: photons

I can only point you in the right direction, because the real answer would be too long. Read some quantum optics books for that.

The idea that photons are tiny particles which form big radio waves is wrong. The whole idea of tiny photons is wrong. The way photons are discussed, is with standing waves or infinite plain waves. A wave like this has a certain energy given by the square of the Poynting vector integrated over the whole wave. How we came to speak of photons were things like the photoemission of electrons or the blackening of points on photographic paper. These are very local events, and it made people believe in a tiny particle colliding with electrons.
But at this point the whole mess becomes apparent: A plain wave is an energy eigenstate of the system. When the photon "hits" the electron the energy of the whole wave needs to disappear. (I am talking about single photons here) But this pretty much happens in one point in space.
Mathematically it is not a big problem, because we just use integral overlaps, but conceptually one needs to move away from the intuitive notion of a particle. Photons as particles live in reciprocal space, and it is much better to see them as the energy units that a plain wave uses to trade its energy.
For some experimental setups like in quantum cryptography it is more useful to see the photon as a wave packet, but this is an approximation which we make because we know that a certain electron has lost energy in a certain time frame, and our free Hamiltonian is an approximation.
I hope I didn't just confuse you more, and there is a lot more to the whole thing, once one starts talking about probabilities and squeezed states. But roughly photons only make sense when we are talking about energy.

4. Nov 26, 2008

### Naty1

Re: photons

Yes. But frequency and density aren't directly related, I don't think.

I would generally agree, but say that to think about photons relationship to waves isn't insightful...doesn't lead to any useful conclusions. Usually what works is to think of light as particles in some situations, as waves in others....

Based on that one word reply,"YES", you can surmise your question has no simple answer...You are entering the world of wave particle duality...and quantum uncertainty....Maybe a photon is a blip, a concentration, of an electromagnetic wave. Or a higher probability of finding a photon in spacetime.

In some cases in the double slit experiment it's "absolutely, positively" a 'particle" in others it's "absolutely, positively" a wave....it's both...you can tease both aspects from it's behaviors...Like how different your girlfriend/boyfriend looks when playing baseball versus going to a nightclub...you'd hardly know its the "same" person.

See my post under the nearby thread "double slit counter" to get a feel for the subtle aspects of photons. Maxwell's equations do a nice job of quantifying electromagnetic waves, yet the photoelectric effect suggests other qualities. Likely you have come across DeBroglie wavelengths which relate matter/mass to wavelike characteristics. Well, the momentum of photons apparently enables them to exhibit wave "particle" duality even without rest mass...(Likely that comment will bring strong objections from purists!)

If a light bulb can give off hundreds of billions of photons why not a radio transmitter?

5. Dec 20, 2008

### xmavidis

Re: photons

That's the way I understand light:

The photon is the particle-picture of light. Photon has momentum but no rest mass.
Light has also a wave-picture with some frequency.

Light is wave and particle simultaneously. Photons are not the elementary parts of light-waves and vice-versa.

6. Dec 23, 2008

### Usaf Moji

Re: photons

Let's say you graph some function. Then you take a very small piece of the graph - from that small piece you have all the info you need to draw the entire graph again.

Think of an EM wave as the whole graph. Now think of a photon as a very small portion of the graph.